This invention is concerned generally with the art of sputtering materials upon substrates and more particularly is concerned with apparatus that is required to sputter at r.f. in the megahertz range. The invention is directed to such apparatus in which it is necessary to transfer large amounts of power from an r.f. source to the sputtering plasma which is formed between anode and cathode electrodes that are mounted in a sputtering chamber and surrounded by background gas.
The reason for the requirement of transferring large amounts of power is that the particular apparatus of the invention is intended to sputter large areas of substrate from target surface that has an area also much greater than conventional. Since the useful power of the sputtering apparatus is invested in the plasma, the larger the plasma area, the larger the target area and the greater the power demands upon the source.
The invention is also concerned primarily with the sputtering of insulating materials in direct sputtering techniques, that is, where the sputtering is effected at r.f. utilizing target means that are formed of the complete compound that is to be sputtered. In such sputtering, the difference in vapor pressure between the different elements making up the compound and the maintenance of stoichiometry in the deposit upon the substrate is insured by the proper type of background gas or gases.
In recent years the art of sputtering nonconductive materials has expanded to a point where the demands which are placed upon the circuitry supplying power to the apparatus have become very great and in addition quite unusual. Conventional methods and circuits which have been disclosed in the prior art are no longer practical and hence cannot enable commercial production of sputtered materials with high yield. In order to sputter large areas of substrate at relatively high speed, one must use large targets and anodes for carrying the substrate so that the plasma is also quite large. Radical changes occur in such apparatus compared to the small throughput laboratory devices where batch sputtering was being carried out upon pieces of substrate that were substantially less than 50 square centimeters in area. Most importantly, for large plasma areas, the electrical phenomena which occur within the pressure chamber are totally different from those which occur in the small chambers and under the conditions known.
The principal problem which arises is brought about because the available power is undesirably lost to the capacitance shunting the plasma rather than absorbed by the plasma in a configuration of the type which includes large electrodes. This will be detailed hereinafter. It is only the power in the plasma which is useful.
Consider the requirements for sputtering a photoconductive material such as cadmium sulfide in a uniform coating upon a continuous strip of substrate such as polyester film or sheet metal that is more than a meter wide. A single target of the material to be sputtered, even for smaller width substrates than one meter, will certainly have an area exposed for generating plasma which is greater than 50 square centimeters and will usually have an exposed area at least several times greater than 50 square centimeters. Furthermore, to increase sputtering speed, such apparatus may have from two to as many as twelve or more large targets whose combined area can be several square meters. The problem which arises because of target size is exacerbated in a device of this size.
It should be kept in mind that the normal configuration for sputtering comprises a cathode and an anode in an atmosphere of gas having heavy ions, as for example, argon, with an electrical field established between them that is at r.f. for the sputtering of insulator materials such as the above-mentioned photoconductor cadmium sulfide. The cathode is the so-called target and is made out of the material to be sputtered, while the anode is the electrode toward which the molecules of the target material are driven for deposit. The substrate overlies the anode and the sputtering plasma which arises when the conditions are correct will appear between the target and the anode; hence the sputtered material will deposit upon the substrate instead of the anode.
For the purpose of aiding in an understanding of the invention herein, attention is invited to several patents which displace the general requirements and the construction of sputtering apparatus which is intended to be utilized for the sputtering of substrate on a production basis. The basic method of laying down a sputtered coating of a photoconductive material of the type for which the invention herein is generally intended is described in U.S. Pat. No. 4,025,339. This patent also discloses in a simplified manner the electrical circuitry of the sputtering configuration and gives an indication of the conditions of sputtering.
It is advantageous to dwell for a moment on the configuration of the electrodes in an arrangement as described in U.S. Pat. No. 4,025,339.
The sputtering occurs by virtue of the creation of an electrical glow discharge plasma between a target and an anode. The target is the cathode and it is coupled to an r.f. source of electrical power which is at an instantaneous negative voltage of several thousand volts. The anode is coupled to the r.f. source as shown in said U.S. Pat. No. 4,025,339 but is at ground potential or close to it. As taught in the said U.S. Pat. No. 4,025,339, the anode may be connected to a potential point that is slightly below ground potential, thereby giving rise to a so-called bias on the anode. While the power is applied in the form of r.f., there is a diode action occurring in the pressure chamber between the electrodes so that it is feasible to discuss the matter as though the arrangement were d.c. in action.
The bias of the anode is a desirable feature of an apparatus and method which enable the sputtered coating of photoconductor to have certain important characteristics as taught in said U.S. Pat. No. 4,025,339 and it must be pointed out that this bias can be achieved in different manners. In some cases it can be effected by direct conductor connections, as illustrated in said U.S. Pat. No. 4,025,339 and in other cases it can be achieved by means of geometry built into the sputtering apparatus and without any physically visible connections. In any event, the invention herein does not revolve around the establishment of a bias on the anode, and even if not present (although preferred), the problems which arise because of the use of large targets are solved irrespective of the presence or absence of bias on the anode. Specifically, if the anode is totally grounded, there will still be difficulty in achieving the sputtering plasma with large targets and the invention is still capable of obviating this difficulty.
In sputtering as contemplated by the invention, the chamber in which the sputtering occurs is formed of metal completely or to a large extent, and for practical and safety purposes, its walls are at ground potential. The target or targets (and there are normally a plurality of these) are mounted to the inner walls of the chamber by suitable insulating means and fittings which are required to provide the necessary mechanical support and at the same time permit of the coupling of r.f. power to the targets. They must also be removable because they are consumed slowly, and often pit and crack and must be replaced.
The anode or anodes are also mounted on the interior of the chamber and, as stated, are at ground or near ground potential. Several forms of sputtering apparatus are disclosed in U.S. Pat. Nos. 3,884,787; 3,829,373; and 4,014,779. From these patents it can be seen that the anode for the production manufacture of sputtered substrate is quite often in the form of a large rotary drum over which the substrate travels as the drum revolves, from a supply to a takeup reel both of which are on the interior of the pressure chamber. Large sputtering machines are not limited in construction to the presence of the supply and takeup on the interior of the chamber but can have air or pressure locks in their walls to enable continuous sputtering upon a pass-through substrate without opening the chamber.
In any sputtering apparatus of the type which is involved in this invention, the chamber is pumped down, the background gases are admitted and the glow discharge plasma is formed between the anode and the target or targets upon application of r.f. power to these electrodes. Discussions of the nature of the gases, their pressures and the techniques are found in the patents which have been mentioned and also in U.S. Pat. No. 3,976,555 which discloses a novel form of target with a special mounting to provide for efficient admission of the background gas and for cooling of the target, which is often required.
When the plasma is established it produces the desired effect by the actual bombardment of the target by heavy ions of the inert or nonreactive gas. These ions literally "splash" molecules of the target material out of the target, the molecules fly across the gap (which is of the order of centimeters) between the target and the anode, and impact against the substrate carried on the anode. The glow discharge plasma comprises the ionized atmosphere of the gas which is usually argon. In normal glow discharge, as in the case of any discharge of ionized gas, there is a dark space immediately at the surface of the target which is probably a form of space charge, this dark space being known as the Crookes dark space. No deposit of material will occur where this dark space is extant. Principally, the molecules will be deposited directly opposite the target so that the facing areas of target and anode control the deposit area, but the plasma does not necessarily confine the path of molecule movement nor is the plasma accurately confined by the subtended facing areas of target and anode.
Because of this fringe movement of high energy target material particles, it is necessary to provide shielding around the target to keep the particles from moving around to its sides and back and to confine the plasma path. This also keeps target material from being deposited on the walls of the chamber, although it is impossible to prevent some such deposit. The shielding is metal and is grounded and spaced from the target by a distance that is approximately the same as the thickness of the Crookes dark space so that in effect the dark space will occur before plasma is produced adjacent the target. This dimension may be several millimeters.
Additionally, as disclosed in the patents which have been mentioned above, the anode is shielded to prevent the deposit of the target material anywhere except upon the substrate that is being moved over the anode. In the case of large drums which are more than a meter long and perhaps almost a meter in diameter, it can be appreciated and understood that the shielding of the anode is quite substantial. This is important to the consideration of the bias of the anode which has been mentioned, but to a small degree upon the problem to which this invention addresses itself.
The frequencies at which sputtering machines may be operated at r.f. are established by the Federal Communications Commission of the United States at this time as the so-called Industrial, Scientific and Medical band (I.S.M.). The frequencies include 13.56 megahertz (mhz) and the second and third harmonics of that frequency. These frequencies are chosen such that operation will result in the least chance of interference with other r.f. services used in the United States. Foreign countries have quite similar arrangements such that the invention herein, as will be apparent, is applicable if used in foreign countries as well.
The apparatus of the invention is intended to be operated at the frequency of 13.56 mhz because there is little or no need for attempting to suppress the second and third harmonics which are those which would have the greatest radiating power. At this frequency, the quarter wavelength is about 5.5 meters, a length which is not impractical for the dimension of shielding members, walls and even certain kinds of electrodes in sputtering apparatus.
It is accepted in the art of sputtering with r.f. that best operation of the apparatus will be achieved if the frequency of operation is above about 6 mhz and the invention thus has particular application in the range of 6 mhz and higher; however, in any practical instance where the frequency of sputtering is effected in the megahertz range, the invention is of substantial value and can be applied. Any frequency below that range does not normally give rise to the problems solved by this invention. At low frequencies, conventional power supply and coupling circuitry can be used to pump power efficiently into a useful load.
It will be understood that the invention as described and claimed is concerned only with apparatus operated in the megahertz range.
Several of the different types of phenomena which give rise to the difficulty of pumping power into the useful load at megahertz frequencies are mentioned hereinafter. The useful load as explained comprises the sputtering plasma which occurs between the targets and the anodes or anode. Because of the fact that power is being robbed by parasitic paths and the like, only a small percentage of power produced at megahertz frequencies which is coupled to the sputtering apparatus by conventional power transfer and matching circuitry is used to produce the plasma.
The phenomena and factors which produce this difficulty are not independent of one another but interact in manners which are almost impossible to analyze on a rational basis. At best, measurements and testing could provide some rough guide as to what is occurring in an apparatus of a construction, but a variation in geometry and size would give rise to completely different factors.
The invention herein provides a structure which is generally suitable for the efficient transfer of power in practically any kind of sputtering apparatus which is designed to coat large areas of substrate from targets of large area.
To return to the discussion of the adverse factors which are involved in the inefficient transfer of power and which are rendered unimportant by the invention, the following are some of the things which give rise to the principal power transfer problems in large sputtering apparatus driven at megahertz frequencies:
A. Large targets and anodes
These would include not only those electrodes which have generally square configurations and configurations departing not too radically from square, but also would include electrodes which are long and thin. The large areas of over 50 square centimeters and more produce large parasitic paths by capacitive coupling to ground through the surrounding structure which includes the actual chamber walls themselves.
The long thin electrodes will additionally introduce distributed inductance along their lengths which becomes quite considerable at the frequencies of operation. If the actual length approaches a quarter wavelength at the frequency of the r.f., resonance and standing waves can complicate matters.
B. Surrounding structures
These would include the shielding around the targets which help in establishing the Crookes dark space around the target, the shielding if any around the anode, the walls of the sputtering chamber and any supporting metalwork in the chamber for other purposes. The presence of the surrounding structures produces capacitive coupling and since the surrounding structure is grounded, there are multiple capacitive bypass paths for the power to ground.
It might be pointed out that it has been noted in some large sputtering apparatus of the type to which the invention is applied that the capacitance of the target and anode electrodes produce approximately ten to twenty percent of the total capacitance estimated for the entire apparatus insofar as input to the load circuit is concerned. This means that the surrounding structure accounts for 80% to 90% of the total capacitance, and all of this is parasitic coupling that is not useful to establish the plasma load.
In one apparatus which has an anode drum with the dimensions of about one meter in diameter and about one meter in length, the coupling of the anode was so insignificant relative to the surrounding structure coupling that the drum could be removed from the apparatus without substantially detuning the feed circuitry of the invention. This was ascertained by using a dummy load of tungsten lamps in place of the plasma. This meant that practically all of the capacitive coupling in the load circuit was provided by other factors, but additionally indicated that the circuit of the invention was effectively achieving the ends sought. A conventional circuit would also look into the same load but would be unable to support the dummy plasma load or an actual plasma load.
C. Power transfer problem generally
If we consider a power source that has an impedance of about 50 ohms and we require this power source to drive a load which looks like 1000 ohms shunted by an impedance to ground of the order of 23 ohms, in order to get the power into the load at a voltage of about 1000 to 2000 volts, the source will have to generate enormous power. To develop 1600 volts RMS across 50 ohms, we would need about 32 kilowatts.
The figures given above represent the actual conditions in a sputtering apparatus which is to be operated in the megahertz range. The r.f. power source has an internal impedance of about 50 ohms; the useful load is the sputtering plasma which has an almost resistive impedance of about 1000 ohms; the impedance effect of the shunting parasitic capacitance to ground is about 23 ohms. Of this 32 kilowatts, about 2000 watts will be used in the target and plasma and the rest of the power will be wasted, reflected back to the source.
What is required is to have an input impedance to the load circuit which matches that of the source. That requirement is impossible to meet with conventional coupling techniques. Thus, if a matching transformer is inserted between the source and the load circuit, the secondary of the transformer would have such a high impedance relative to the shunting capacitance that the greatest majority of power would be displaced in the secondary itself. What is not dissipated would be reflected back to the source of r.f. and the load would get little if any power.
D. Resonance in matching components
This problem is probably related to the previous subject matter because it is concerned with impedance matching. In an impedance matching circuit of the type which would be considered conventional to achieve the power transfer needed, it is required to self-resonate parallel circuits. In the megahertz frequency range, and especially at 13.56 mhz which is the preferred frequency, it is very difficult if not impossible to achieve self-resonance using practical components. The inductance required to resonate with the high capacitance of the parasitic paths would have to be of the order of a couple of hundred nanohenries, and an inductor having this minute inductance will be unable to handle the required power, let alone be capable of ready construction.
In order to follow the description of the invention, it is essential to understand and appreciate the concept of the factor Q or figure of merit or quality of a component or a circuit made up of electrical parts. It is also essential to understand the difference between the static factor Q and the dynamic or loaded factor Q which will be designated QL hereinafter.
Initially, it may be generally understood that the quality factor Q is a ratio of the energy storing ability of a component or circuit to its energy loss.
Unloaded or static Q should normally be as high as possible. In the case of inductors, this is achieved by using materials that produce as little resistance as possible, as for example by using high conductivity tubing. In the case of the coils of the inductors of the invention which will be described, some of them are wound from copper tubing which is upwards of 8 mm. in diameter and silver-plated. Capacitors should have the lowest loss possible, and in the case of the capacitors used in circuits according to the invention, these are vacuum dielectric type, the variable ones being varied by mechanical shafts passing through vacuum seals.
A typical coil for use with a tuned circuit of the invention having four turns of 13 mm. diameter copper tubing in a helix about 15 centimeters long and 15 centimeters in diameter is tapped at suitable locations inwardly from its ends and tuned to resonance by a vacuum capacitor rated at 20,000 volts and having a capacitance which can be varied from 12 to 100 picofarads. Such a coil would have a static Q of several hundred or more.
Loaded or dynamic Q is a totally different matter because of circulating current. In the case of resonant circuits such as those of the invention, QL is determined generally by the relationship between the circulating current in the circuit and the current to be supplied by the input source to the circuit. High QL means high circulating current or high impedance and this is not desirable for maximum power transfer. It then becomes a compromise between the dynamic QL and the static Q. The static Q should be as high as possible and the QL should be quite low.
In tank circuits of the type involved in the invention, it has been accepted in the art that efficient power transfer is achieved where the QL of the tank circuit is preferably below 15. The range which is usually satisfactory is between 3 and 12 although that upper limit is not preferred. This range provides good stability for tuning since the response is broader than with higher values of QL. Higher values of QL increase the impedance of the circuit and increase the circulating current. Incidentally, for transmission and reception of intelligence, the broadening effect of low QL is undesirable because it decreases the ability of a given circuit to reject unwanted signals.
In the case of intelligence transmission, power transfer by tuned circuits is desired as well as in the case of a sputtering apparatus, but the purposes are different. In the case of an antenna, it is desired to produce the greatest current possible in the load, that is, the antenna. In the case of a sputtering plasma, it is desired to produce the greatest voltage possible in the load.
The targets of the invention have capacitances of the order of 250 to 650 picofarads so that the effective dynamic resonant load impedance of a tuned circuit must include the very substantial parasitic capacitance and some external parallel inductance chosen to resonate at the exciting frequency. This impedance turns out to be of the order of a few hundred ohms. But it is essential to develop a moderately high voltage of the order of 2,000 to 4,000 volts peak to peak across the tuned circuit.
By simple calculations using typical values encountered in the apparatus which is the subject matter of this invention, it can be seen that there is a typical problem. Assuming the source to be a 50 ohm 13.56 megahertz r.f. source, if the typical target has a parasitic capacitance of about 350 picofarads (0.00035 microfarads) the operating values will assume proportions making the target circuit extremely difficult to drive from the source with any reasonable efficiency of power transfer. This is shown as follows:
Assume that the target capacitance which is all parasitic is 350 pf, then ##EQU1## where
X.sub.CT is the capacitance reactance of the target,
f is the r.f. frequency and
CT is the target capacitance.
The value of QL can now be computed from a well-known formula and the value of the plasma resistance which, for a typical apparatus of the size involved will be approximately 1,000 ohms. This value is ##EQU2## where QL is the loaded figure of merit or quality which was discussed above and X.sub.CT is the same parameter used in equation (1). The target impedance ZT is the product of the quality factor QL and the target impedance X.sub.CT.
As noted, the value of QL given in equation (2) is much too large for efficient power transfer, being almost twice the maximum mentioned.
A conventional solution to this dilemma is to connect the source through a center tapped coil at the target. The resulting effect on power transfer can be ascertained from the following: ##EQU3## where
n1 is the number of turns in the primary winding of the coil, input to ground;
n2 is the number of turns in the secondary winding of the coil, input to output;
nt is the total number of turns in the coil;
X.sub.CT (eff) is the effective target capacitive reactance; and
X.sub.CT is the same parameter as used in equations (1) and (2).
In equation (3) it is assumed that the center tap is at the exact center of the coil, which is typical.
Computing the target resistance for the center-tapped circuit, ##EQU4## where RT.sub.A is the effective target resistance.
We can now compute QL for this center tapped circuit using the formula from equation (2) and equations (3) and (4). ##EQU5## and substituting in the formula given above for the target impedance we see. EQU ZT(eff)=QL.sub.A .multidot.X.sub.CT =500 ohms (6)
here ZT(eff) is the effective impedance of the target circuit using the center tapped coil.
If we assume that the input to the tapped center coil is the 50 ohm source described above, then we can determine the location of the tap of the resonant coil in order to achieve the correct impedance match on the basis of the following: ##EQU6## where
T.sub.1N is the number of turns at the proper tap and
Z.sub.1N is the impedance at that tap.
For the assumed values this turns out to be the 31.6% point which gives a voltage step-up of 3.16 to 1. With a typical power capacity of the source being 1,800 watts, 300 volts RMS will be developed across the 50 ohms of the output of the source but only 948 volts RMS will appear across the target circuit, this being 2,675 volts peak to peak. This is an efficiency of about 74.3% and the voltage is insufficient to provide the necessary sputtering effect. This requires a peak to peak voltage at the target which is upwards of 3,600 volts peak to peak. In order to achieve this with the conventional tapped coil as described above, the power source would have to be increased to about 2,400 watts and of this energy, about 600 watts will be lost in useless heat and reflected back to the source during operation.
As will be detailed hereinafter, the invention achieves an efficiency of more than 90% because of the circuitry which will be described and claimed. The 1,800-watt power source mentioned will achieve more than 3,600 volts peak to peak at the target.
As will be further described, the invention is ideally suited for multiple target apparatus whereas the conventional circuit becomes worse. When the number of targets is increased and these are driven in parallel, the impedance decreases by a factor which is equal to the number of targets, the actual effect being an increase in capacitance. If we assume that the best coupling efficiency for a conventional tapped circuit which is described above is about 74%, by the time the number of targets has increased to twelve (which is typical of the apparatus of the invention) the efficiency will have dropped to about 22%. In such a case, for an actual 1,800 watts of power per target, the conventional system would require a source of 100,700 watts with a resultant loss of about 80,000 watts.
On the other hand, the efficiency of the invention is not materially affected. By slight adjustment, the circuit of the invention continues to drive the twelve targets from a source of suitable wattage, say about 22 KW with an efficiency of well over 90% and very little losses.